KR20150034210A - Hardened steel tube member, automobile axle beam using hardened steel tube member, and method for manufacturing hardened steel tube member - Google Patents

Abstract

The quenched steel pipe member is formed of a GI galvanized steel pipe and has a cross section perpendicular to the longitudinal direction at a central portion in the longitudinal direction of the GI galvanized steel pipe with a contact portion in contact with the inner circumferential surfaces of the GI galvanized steel pipe And the contact portion is integrated by the Fe-Zn alloy phase, and the micro Vickers hardness at the depth of 50 mu m from the surface of the base material is 95% or more of the micro Vickers hardness at the depth of 200 mu m from the surface of the base material. %.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an axle beam for an automobile and a method of manufacturing a steel pipe for an automobile using the quenching steel pipe member,

The present invention relates to a quenching steel pipe member, a vehicle axle beam using the quenching steel pipe member, and a manufacturing method of the quenching steel pipe member. The present application claims priority based on Japanese Patent Application No. 2012-207249 filed on September 20, 2012, the contents of which are incorporated herein by reference.

The automobile axle beam is a member connecting the right and left axles, and a repeated load is applied during running, so that high fatigue characteristics are required.

Therefore, as disclosed in Patent Document 1, there has been proposed a method of manufacturing an axle beam for automobiles in which a steel pipe is press-formed and quenched to increase the strength of the steel pipe to secure fatigue characteristics.

However, in this method, since the furnace is quenched, the heating time is long, the outermost surface of the member is decarburized and softened, and sufficient fatigue characteristics can not be obtained.

In order to suppress this surface layer softening, there is proposed a technique of forming a carbon-enriched layer on the surface of a steel material by curing the surface of the steel pipe after quenching by performing galvanization on the surface of the steel pipe as disclosed in Patent Document 2 .

However, this method requires heating for a long time because heating is performed in the furnace, and zinc is volatilized in the meantime. Therefore, there is a problem that zinc is plated extra, including volatilized ones, resulting in a huge cost.

Patent Document 3 discloses an axle beam having excellent fatigue characteristics obtained by press-molding a steel pipe into a V-shaped section in a predetermined press condition.

However, the patent document 3 aims at providing an axle beam that can exhibit excellent fatigue characteristics even without performing heat treatment such as quenching, and the decarburization of the outermost surface by the heat treatment as described above is not mentioned at all It is not.

Japanese Patent Application Laid-Open No. 2005-171337 Japanese Patent Application Laid-Open No. 2006-45592 Japanese Patent Application Laid-Open No. 2009-274077

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method of manufacturing a quenched steel pipe member, an automobile axle beam and a quenched steel pipe member which are excellent in fatigue characteristics and are low in cost.

The outline of the present invention is as follows.

(1) According to a first aspect of the present invention, there is provided a GI galvanized steel pipe which is formed of a GI galvanized steel pipe and has a cross section perpendicular to the longitudinal direction at a central portion in the longitudinal direction of the GI galvanized steel pipe, And the micro-Vickers hardness at a depth of 50 mu m from the surface of the base material layer is smaller than the micro-Vickers hardness at a depth of 200 mu m from the surface of the base material layer. Is a quenched steel pipe member having 95% or more of micro Vickers hardness.

(2) In the quenched steel pipe member according to (1), the micro Vickers hardness of the GI galvanized steel pipe at a depth of 50 mu m from the base material surface layer may be 500 Hv or more.

(3) In the quenched steel pipe member according to (1) or (2), the contact portion may be formed over a length of at least 50% of the entire length of the GI galvanized steel pipe.

(4) The second aspect of the present invention is an automobile axle beam using the quenched steel pipe member according to any one of (1) to (3).

(5) According to a third aspect of the present invention, there is provided a GI galvanized steel pipe, wherein a cross section perpendicular to the longitudinal direction at the center in the longitudinal direction of the GI galvanized steel pipe is in contact with the inner circumferential surfaces of the GI galvanized steel pipe (G / m < 2 >), a maximum heating temperature T (DEG C) of 850 DEG C or more, and a maximum heating temperature holding time t (hr) A heating and holding step of heating and holding the press-molded GI galvanized steel pipe under the condition that the following formula (I) is satisfied; and a step of cooling the heated and maintained GI galvanized steel pipe by water- And a cooling step of integrating the steel pipe with the steel pipe.

(6) In the method of manufacturing a quenched steel pipe member according to (5), in the cooling step, the GI galvanized steel pipe heated and held may be water-cooled to 200 ° C or lower at a cooling rate of 30 ° C / s or higher.

(7) In the method of manufacturing a quenched steel pipe member according to (5) or (6), the zinc plating amount A may be 60 g / m 2 or more.

(8) In the method of manufacturing a quenched steel pipe member according to any one of the above items (5) to (7), in the press molding step, the contact portion has a length of 50% or more of the entire length of the GI- The GI galvanized steel pipe may be formed by press-molding.

(9) In the method of manufacturing a quenched steel pipe member according to any one of (5) to (8), the GI galvanized steel pipe may be energized and heated in the heating and holding step.

(10) In the method of manufacturing a quenched steel pipe member according to any one of (5) to (9), in the heating and holding step, the press-molded GI galvanized steel pipe is heated to a temperature of 3 Sec or more and 30 seconds or less.

(11) In the method of manufacturing a quenched steel pipe member according to any one of (5) to (10), the GI galvanized steel pipe may have a component system having Ac3 point of 850 캜 or lower.

According to the quenched steel pipe member described above, since it is formed by the GI galvanized steel pipe, decarburization of the surface layer is suppressed by zinc plating, high surface hardness can be secured, and fatigue characteristics are improved.

Further, since the contact portions where the inner circumferential surfaces of the GI galvanized steel tubes are in contact with each other are integrated by the Fe-Zn alloy phase, deterioration of the fatigue life due to friction of the contact portions can be suppressed and fatigue characteristics are improved. As a result, it is possible to reduce the thickness of the thin film, thereby significantly reducing the cost.

According to the manufacturing method of the quenched steel pipe member described above, the GI galvanized steel pipe is heated and maintained under the condition satisfying the formula (I) to suppress the decarburization of the surface layer with the minimum amount of zinc plating, It is possible to control decarburization of the surface layer. Therefore, the cost can be greatly reduced.

1A is a plan view of an automobile axle beam according to the present embodiment.
1B is a perspective view of the axle beam for the automobile.
1C is a sectional view of the IC-IC in Fig. 1A.
FIG. 1D is a cross-sectional view of the ID-ID of FIG. 1A.
2 is a process explanatory view for manufacturing a quenched steel pipe member according to the present embodiment.
3 is a state diagram of an Fe-Zn alloy.

Hereinafter, an automobile axle beam (hereinafter referred to as an axle beam) according to an embodiment of the present invention will be described in detail. In the following description, the axle beam is used as a concrete example of the quenched steel pipe member. However, the quenched steel pipe member according to the present invention is not limited to this, and a high fatigue characteristic such as a structural member for industrial machinery, And various quenched steel pipe members.

1A and 1B are a plan view and a perspective view showing an axle beam 1 according to the present embodiment. 1C is a sectional view of the IC-IC of FIG. 1A, and FIG. 1D is a sectional view of the ID-ID of FIG. 1A.

As shown in Figs. 1A and 1B, an axle beam 1 according to the present embodiment has a GI galvanized steel pipe 10 having a cross section perpendicular to the longitudinal direction (hereinafter referred to as a vertical cross section) By press molding so as to be in a shape.

As shown in Fig. 1C, the axle beam 1 according to the present embodiment has a contact portion 11 in which the inner circumferential surfaces of the axle beam 1 come into contact with each other at the central portion in the longitudinal direction.

The contact portion 11 is integrated with the Fe-Zn alloy phase by performing the quenching treatment in a state in which the zinc plating on the inner circumferential surface of the GI galvanized steel pipe 10 is in contact with each other.

According to this configuration, in addition to the effect of increasing the rigidity of the axle beam 1, it is possible to suppress the fatigue life degradation due to the friction between the inner circumferential surfaces of the GI galvanized steel pipe 10, and to improve the fatigue characteristics.

In addition, as shown in Fig. 1 (d), the inner circumferential surfaces do not need to be in contact with each other at the portions spaced apart from the central portion in the longitudinal direction. That is, the contact portion 11 only needs to be in contact with the GI galvanized steel pipe 10 at the center in the longitudinal direction thereof.

However, it is preferable that the contact portion 11 is formed over a length of at least 50% of the entire length of the GI galvanized steel pipe 10 in order to more suitably exhibit the effect of improving the fatigue characteristic, It is more preferable that it is formed over the entire surface.

Further, since the axle beam 1 according to the present embodiment is obtained by subjecting the GI galvanized steel pipe 10 to the quenching treatment after press forming, the quenching treatment is carried out while suppressing decarburization from the surface layer by the effect of zinc plating So that the hardness of the entire axle beam 1 can be increased. That is, in the surface layer portion of the axle beam 1, decarburization is suppressed, so that the hardness equivalent to the central portion of the plate thickness (thickness) can be secured, thereby improving the fatigue characteristic.

More specifically, in the axle beam 1 of the present embodiment, the micro-Vickers hardness at the depth of 200 mu m from the surface of the base material is defined as X, the micro-Vickers hardness at the depth of 50 mu m from the surface of the base material is defined as Y, / X) x 100 is 95 or more. The micro Vickers hardness is measured at a load of 50 g.

If the value of (Y / X) x 100 is less than 95, the fatigue life may be lowered due to fatigue cracks from the surface layer. (Y / X) x 100 is preferably 96 or more, and more preferably 97 or more.

The micro Vickers hardness Y at a depth of 50 mu m from the surface of the base material is preferably 500 Hv or more in terms of micro Vickers hardness in order to secure high fatigue characteristics, and more preferably 540 Hv or more.

As described above, according to the axle beam 1 of the present embodiment, since the GI galvanized steel pipe 10 is formed by the galvanized steel pipe 10, decarburization of the surface layer is suppressed by the effect of galvanizing and the hardness of the surface layer is improved, Since the contact portions 11 where the inner circumferential surfaces of the GI galvanized steel pipe 10 are in contact with each other are integrated by the Fe-Zn alloy phase, the reduction in the fatigue life due to the friction between the inner circumferential surfaces can be suppressed And the fatigue characteristics can be synergistically improved. As a result, it is possible to reduce the thickness of the thin film, thereby significantly reducing the cost.

In the present invention, the chemical composition of the steel material of the GI galvanized steel pipe 10 is not particularly limited, but preferable composition of the steel is described. Hereinafter,% with respect to the content of chemical components means% by mass.

The chemical composition of the steel material of the GI galvanized steel pipe 10 may contain C, Si, Mn, Ti, and B in the following ranges.

C: 0.15 to 0.30%

C is an element that determines the strength of the axle beam 1. The C content is preferably 0.15% or more, and more preferably 0.20% or more, in order to secure the strength for obtaining sufficient fatigue characteristics. In order to set the hardness to Hv 500 or more, it is preferable to be 0.24% or more. Further, in order to suppress the occurrence of the quenching crack, the C content is preferably 0.30% or less, and more preferably 0.25% or less.

Si: 0.05 to 0.35%

Si is a deoxidizing element and also contributes to solid solution strengthening. In order to obtain these effects, it is preferable to contain 0.05% or more. In addition, by setting Si to 0.35% or less, toughness can be ensured. The lower limit of the Si content is more preferably 0.20%, and the upper limit of the Si content is more preferably 0.30%.

Mn: 0.5 to 2.0%

Mn is an element which improves the quenching property, and Mn content is preferably set to 0.5% or more, because the effect of improving the hardenability can be sufficiently secured. By setting the Mn content to 2.0% or less, the deterioration of the delayed fracture characteristics can be suppressed, the precipitation of MnS can be suppressed, and the fatigue strength in the vicinity of the welded joint can be avoided.

The lower limit of the Mn content is more preferably 1.0%, and the upper limit of the Mn content is more preferably less than 1.7%.

Ti: 0.005 to 0.05%

Ti serves to stabilize and effectively improve the quenching by the addition of B by suppressing the precipitation of BN by fixing N in the steel as TiN. Therefore, it is preferable to add at least 3.42 times of the N content so as to meet the stoichiometry of TiN, and the preferable range of the Ti content is automatically determined from the range of the N content.

However, since some carbides precipitate, it is preferable to set the range of 0.005 to 0.05%, which is slightly higher than the theoretical value, in order to more secure N fixing. More preferably, it is 0.01 to 0.02%.

B: 0.0005 to 0.005%

B is an element that significantly improves the quenching of the steel by adding a small amount. When the B content is set to 0.0005% or more, an effect of improving the quenching property is suitably obtained, and more preferably 0.001% or more.

When the content of B is 0.005% or less, the formation of coarse B-containing precipitates can be suppressed and brittleness can be suppressed, and more preferably 0.002% or less.

The steel chemical composition of the GI galvanized steel pipe 10 may be limited to the following ranges of Al, P, S, N, and O.

Al: 0.08% or less

Al is an element useful as a deagglomeration material of molten steel, and it is preferable to add Al by 0.01% or more. Since Al is also an element that fixes N, the amount of Al greatly affects the crystal grain size and mechanical properties. By setting the Al content to 0.08% or less, occurrence of surface scratches on the product due to non-metallic inclusions can be suppressed, which is preferable. The Al content is more preferably 0.05% or less.

P: not more than 0.05%

P is an element which adversely affects the weld cracking resistance and toughness of the steel, and therefore it is preferably 0.05% or less, and more preferably 0.03% or less.

S: Less than 0.0030%

S deteriorates toughness and precipitates out MnS to lower the fatigue strength in the vicinity of the welded joint, so that the S content is preferably less than 0.0030%, more preferably 0.0026% or less.

In order to suppress the precipitation of MnS, it is preferable to suppress the Mn content in relation to the Mn content, not to suppress only the S content. More specifically, it is preferable that the product of the Mn content and the S content is 0.0025 or less Do. By setting the value of the product of the Mn content and the S content to 0.0025 or less, the fatigue strength in the vicinity of the fully welded portion can be sufficiently secured.

N: not more than 0.006%

N is an element having an effect of precipitating nitrides or carbonitrides to increase strength. However, in the case of the B-added steel, the hardness due to the precipitation of BN is reduced. However, as described above, the addition of Ti to prevent precipitation of BN causes a decrease in hot workability and fatigue strength due to precipitation of TiN, There is a problem of deterioration of toughness. On the other hand, TiN has an effect of suppressing coarsening of the? Particle size at a high temperature and improving toughness. Therefore, in order to optimize the balance of hot workability, fatigue strength and toughness, the N content is preferably 0.006% or less. The N content is more preferably 0.001 to 0.005%, and still more preferably 0.002 to 0.004%.

O: not more than 0.004%

O is CaO, which is an element which deteriorates the effect of Ca addition. Therefore, the O content is preferably limited to 0.004% or less.

The steel chemical composition of the GI galvanized steel pipe 10 may contain at least one of Mo, Cr, Nb, V, and Ni as optional elements, if necessary, in the following ranges.

Mo: 0.05 to 0.5%

Mo is an element having an effect of improving quenching. If the Mo content is less than 0.05%, these effects can not be sufficiently expected. On the other hand, if the Mo content exceeds 0.5%, the alloy cost increases, and therefore, the Mo content is preferably set within the range of 0.05 to 0.5%.

Cr: 0.05 to 1.0%

Cr is not an essential addition element but is an element added for the purpose of improving the quenching property. The Cr content is preferably 0.05% or more, more preferably 0.10% or more, in order to sufficiently obtain the improvement effect of the hardness. Further, it is preferable that the Cr content is 1.0% or less from the viewpoint of suppressing the occurrence of defects during welding, more preferably 0.8% or less.

Nb: 0.01 to 0.1%

Nb has an effect of enhancing precipitation strengthening by Nb carbonitride and also has an effect of improving the toughness by reducing the crystal grain size of the steel material. When the Nb content is 0.01% or more, the effect of improving the strength and toughness can be sufficiently obtained. On the other hand, even if the content of Nb is more than 0.1%, the improvement effect can not be expected further and the cost is increased. Therefore, the content of Nb is preferably in the range of 0.01 to 0.1%.

V: 0.01 to 0.1%

V is an element having an effect of precipitation strengthening by V carbonitride. By setting the V content to 0.01% or more, these effects can be suitably exhibited, which is preferable. On the other hand, even if the V content is contained in excess of 0.1%, the improvement effect can not be expected further, and the alloy cost is only increased. Therefore, the V content is preferably 0.1% or less.

Ni: 0.1 to 1.0%

Ni is an element having an effect of improving quenching and toughness. When the Ni content is 0.1% or more, the effect can be suitably exhibited. On the other hand, if the Ni content exceeds 1.0%, the alloy cost increases, and therefore, the Ni content is preferably 1.0% or less.

That is, the steel chemical composition of the GI galvanized steel pipe 10 contains C, Si, Mn, Ti and B within the above-mentioned range, and Al, P, S, N and O are limited to the above- At least one of Mo, Cr, Nb, V and Ni may be contained in the above-mentioned range if necessary, and the remaining part may be a chemical component composed of Fe and unavoidable impurities.

Further, in the present invention, in order to make the structure of the axle beam 1 into martensite by quenching, it is necessary to sufficiently secure the quenching of the material. As an index of the hardness, for example, a critical cooling rate Vc90 (占 폚 / s) conventionally known by "Iron and Steel, 74 (1988) P.1073" may be used. This is an index represented by the following formula (A), which means a cooling rate at which the volume ratio of martensite becomes 90% or more. Therefore, the lower the Vc90 is, the higher the quenching is, and the martensite structure is obtained even if the cooling rate is slowed down.

However,? = 2.7C + 0.4Si + Mn + 0.8Cr + 2.0Mo + 0.45Ni.

Further, when B (boron) is not contained, (Formula A) is changed to (Formula A ').

However,? '= 2.7C + 0.4Si + Mn + 0.8Cr + Mo + 0.45Ni.

Next, the manufacturing method of the axle beam 1 will be described in detail.

As shown in the flowchart of Fig. 2, the method of manufacturing the axle beam 1 according to the present embodiment has at least a press forming step, a heating holding step, and a cooling step. Hereinafter, each step will be described in detail.

(Press forming step)

First, in the press-forming step, the GI galvanized steel pipe 10 is press-molded in a substantially V-shape by giving a displacement toward the inside from the outside along the longitudinal direction to obtain the shape of the axle beam 1. That is, with respect to the GI galvanized steel pipe 10, a cross section perpendicular to the longitudinal direction at the central portion in the longitudinal direction thereof is a substantially V-shape including a contact portion 11 in contact with the inner circumferential surfaces of the GI galvanized steel pipe 10 And press-formed to have a shape.

1A to 1D. As shown in Fig. 1C, at the central portion in the longitudinal direction, a contact portion 11 in which the inner surfaces of the steel pipe are in contact with each other is formed. Both ends of the steel pipe are flat and crushed.

(Heating and holding process)

In the heating and holding process, the GI galvanized steel pipe 10 thus press-molded is heated and held under the condition satisfying the following expression (1).

(A) is the zinc plating amount (g / m 2) of the galvanized steel pipe 10, T is the maximum heating temperature (캜) of 850 캜 or more and t is the maximum heating temperature holding time (hr) . In the present specification, (T + 273.15) x (logt + 20) relating to the heating condition is referred to as a heat treatment parameter B.

By designing the zinc plating amount A and the heat treatment parameter B so as to satisfy the expression (1), the micro Vickers hardness at the depth of 200 mu m from the surface of the base material is represented by X and the micro Vickers hardness at the depth of 50 mu m from the surface of the base material is represented by Y, (Y / X) x 100 can be set to 95 or more.

Further, since the maximum heating temperature is 850 DEG C or higher, the quenched structure can be made into martensite. On the other hand, if the maximum heating temperature is lower than 850 deg. C, it becomes a two-phase region, and a portion which is not quenched is generated, and the fatigue characteristic is rapidly lowered.

That is, by heating and maintaining the GI galvanized steel pipe 10 under the condition satisfying the above-mentioned formula (1), the fatigue characteristics can be improved by suppressing surface decarburization and the fatigue characteristics It is possible to enhance the fatigue characteristics synergistically.

The zinc plating amount A of the GI galvanized steel pipe 10 is preferably 60 g / m 2 or more from the viewpoint that the surface decarburization in the heating and holding process can be more reliably suppressed.

As the heating method in the heating and holding step, for example, energization heating, induction heating, and heating can be used. However, in consideration of productivity, electrification heating is more preferable.

The upper limit of the maximum heating temperature is not specifically defined, but zinc may be volatilized from the surface of the steel pipe if the temperature is excessively high. Therefore, in order to more reliably suppress the surface decarburization, the upper limit may be set at 1100 deg.

The holding time is preferably 3 seconds or longer in the temperature range of Ac3 point or more. If the holding time is 3 seconds or more, the temperature deviation can be more reliably suppressed, and the hardness deviation after quenching can be reduced. Further, since the iron can be reliably diffused into the zinc plating layer, the contact portion 11 can be integrated with the Fe-Zn alloy phase stably.

The holding time is preferably 30 seconds or less. By keeping the holding time to 30 seconds or less, excessive diffusion of iron into the zinc plated layer can be suppressed.

When the GI galvanized steel pipe 10 is heated by conduction heating, it is feared that current drifts and a temperature deviation occurs. However, when the holding time is set to 3 to 30 seconds, it can be heated to a temperature quenching uniformly I could.

As described above, in the method of manufacturing the axle beam 1 according to the present embodiment, in order to heat and maintain at the maximum heating temperature of 850 DEG C or more, the temperature range where the Ac3 point of the steel material is kept as low as possible is expanded, It is preferable to use a steel tube of a component system whose Ac3 point is 850 DEG C or lower.

Here, Ac3 point can be calculated by the following expression (2).

The component system of the steel having the quenching property (Vc90) of 70 DEG C / sec or less can be represented by the following formulas (3) and (4). Vc90 means a cooling rate at which 90% or more of the martensite becomes the martensite.

When the GI galvanized steel pipe 10 is kept under the above-described conditions, the galvanized phase on the surface of the steel pipe enters the region to which hatching is applied in the state diagram of Fig. 3, The contact portions 11 of the contact portions 11 are integrated. As a result, the fatigue life due to the friction between the inner circumferential surfaces of the GI galvanized steel pipe 10 can be suppressed, and the fatigue strength is improved. As described above, if the diffusion of iron into the plating phase is insufficient, it is shifted to the left side of the hatched region, which is not preferable because the iron-zinc alloy phase and the iron are mixed with each other even if quenched.

(Cooling process)

In the cooling step, the hot-held GI galvanized steel pipe 10 is water-cooled, and the contact portions 11 are integrated by the Fe-Zn alloy phase.

In the cooling step, cooling is preferably performed at a cooling rate of not less than 30 DEG C / s until the temperature reaches 200 DEG C or less, because the fatigue characteristics can be further improved by martensitization. It is more preferable that the cooling rate is 50 DEG C / s or more.

The cooling method may be spray cooling, submerged cooling, water cooling, or the like, but spray cooling is preferable in view of productivity.

The axle beam 1 according to the present embodiment obtained in this manner is obtained by the fact that the contact portions 11 are integrated by the Fe-Zn alloy phase and the micro Vickers hardness at the position 50 mu m deep from the base material surface layer is 200 The fatigue characteristics can be synergistically improved by the effect of improving the fatigue characteristics due to the suppression of the surface layer decarburization and the effect of improving the fatigue characteristics due to the alloying of the contact portions 11 .

The reason why the galvanizing is performed instead of the GA is that the GA is already alloyed, so that the state of the GI is the same as that in which the GI is heated for a long time even in a short heating period, and after the cooling, the iron- And the effect of integrating the contact portions 11 by the Fe-Zn alloy phase becomes insufficient.

Example

Examples are shown below.

A GI steel sheet having a composition of 0.24% C-0.2% Si-1.2% Mn-0.02% Ti-10 ppm B was welded as a whole in Examples 1 to 6 and Comparative Examples 1 to 3 and then subjected to press working, To measure the micro-Vickers hardness at a depth of 50 mu m from the surface of the base material, and the micro-Vickers hardness and fatigue characteristics at a depth of 200 mu m from the surface of the base material. With respect to Inventive Examples 1 to 6 and Comparative Examples 1 and 2, the contact portions were integrated by alloying.

Table 1 shows various setting conditions and measurement results. A is the plating amount (g / m 2), T is the maximum heating temperature (° C.), t is the holding time (hr), B is the heat treatment parameter, X is the micro Vickers hardness at the depth of 200 μm from the surface layer of the base material, Micro-Vickers hardness at a depth of 50 mu m.

In Inventive Examples 1 to 6 in which the value of B / A satisfies the range of the present invention, the value of Y / X can be made 95% or more by the effect of suppressing the decarburization from the surface layer, High fatigue characteristics can be obtained by the synergistic effect with the fatigue characteristic improving effect by the fatigue cracking. With respect to Inventive Examples 1 to 5 in which the micro Vickers hardness at the depth of 50 mu m from the surface layer of the base material was 500 Hv or more in Examples 1 to 6, the fatigue characteristics were further improved as compared with Inventive Example 6. [

On the other hand, in Comparative Examples 1 and 2 in which the value of B / A did not satisfy the range of the present invention, the hardness of the surface layer portion was lowered by decarburization from the surface layer, and the effect of improving the fatigue characteristics A synergistic effect could not be obtained and high fatigue characteristics could not be obtained.

In Comparative Example 3 in which plating was not carried out, not only the hardness of the surface layer portion was lowered by decarburization from the surface layer but also the effect of improving the fatigue characteristics due to alloying of the contact portions was not obtained, .

As apparent from the above results, according to the present invention, it is possible to obtain an axle beam having an excellent fatigue characteristic and a low cost.

According to the present invention, it is possible to provide a method of manufacturing a quenched steel pipe member, an automobile axle beam, and a quenched steel pipe member that are excellent in fatigue characteristics and at low cost.

1: Axle beam
10: GI galvanized steel pipe
11:
A: Zinc plating amount
B: Heat treatment parameter
t: retention time
T: Maximum heating temperature
X: Micro Vickers hardness at a depth of 200 μm from the surface of the base material
Y: Micro Vickers hardness at a depth of 50 μm from the surface of the base material

Claims (11)

GI galvanized steel pipe,
Wherein a cross section perpendicular to the longitudinal direction at a central portion in the longitudinal direction of the GI galvanized steel pipe has a substantially V shape including a contact portion where inner circumferential surfaces of the GI galvanized steel pipe are in contact with each other,
The contact portion is integrated with the Fe-Zn alloy phase,
Wherein a micro Vickers hardness at a depth of 50 mu m from the surface of the base material is 95% or more of a micro Vickers hardness at a depth of 200 mu m from the surface of the base material. The method according to claim 1,
Characterized in that the micro Vickers hardness of the GI galvanized steel pipe at a depth of 50 mu m from the base material surface layer is 500 Hv or more. 3. The method according to claim 1 or 2,
Wherein the contact portion is formed over a length of at least 50% of the entire length of the GI galvanized steel pipe. An axle beam for an automobile, characterized by using the quenched steel pipe member according to any one of claims 1 to 3. The GI galvanized steel pipe was press-molded so that a cross section perpendicular to the longitudinal direction at the center in the longitudinal direction of the GI galvanized steel pipe had a substantially V shape including a contact portion in contact with the inner circumferential surfaces of the GI galvanized steel pipe A press forming step for forming a press-
Under the condition that the zinc plating amount A (g / m 2), the maximum heating temperature T (° C) of 850 ° C. or more and the maximum heating temperature holding time t (hr) satisfy the following formula (1) A heating and holding step of heating and holding the steel pipe,
And cooling the heated and maintained GI galvanized steel pipe by water cooling to integrate the contact portion with the Fe-Zn alloy phase.

6. The method of claim 5,
In the cooling step, the GI galvanized steel pipe heated and held is water-cooled to 200 ° C or lower at a cooling rate of 30 ° C / s or higher. The method according to claim 5 or 6,
Wherein the zinc plating amount A is not less than 60 g / m < 2 >. 8. The method according to any one of claims 5 to 7,
Wherein the GI galvanized steel pipe is press-molded so that the contact portion is formed over a length of 50% or more of the entire length of the GI galvanized steel pipe in the press molding step. 9. The method according to any one of claims 5 to 8,
Wherein the GI galvanized steel pipe is heated by energization in the heating and holding step. 10. The method according to any one of claims 5 to 9,
Characterized in that in the heating and holding step, the press-formed GI galvanized steel pipe is energized and heated so as to remain in a temperature region of Ac 3 point or more of the steel for 3 seconds or more and 30 seconds or less. 11. The method according to any one of claims 5 to 10,
Wherein the GI galvanized steel pipe has a component system having Ac3 point of 850 DEG C or less.

Images